The purpose of this study is to help elucidate the molecular mechanisms by which Ca2+/sarcolemmal interactions may be involved in the regulation of excitation-contraction (E-C) coupling in cardiac muscle. This study will concentrate upon two interdependent areas: A) An analysis of the interactions of mono- and divalent cations with the sarcolemma in terms of zeta potential, surface charge densities, and Gouy-Chapman-Stern theory to: 1) help distinguish between the potential contributions of the diffuse double layer and cation/phospholipid binding in Ca2+/sarcolemmal interactions, and 2) to test the hypothesis that Ca2+ bound to the sarcolemma functions as a mobile Ca2+ reservoir which is utilized during muscle contraction. B) An examination of sarcolemmal lipid/lipid interactions to help ascertain whether Ca2+/lipid interactions in the membrane are: 1) due to the primary, independent binding of Ca2+ to negatively charged phospholipids in the membrane, or 2) are a secondary function of steric constraints, as well as charge effects, due to the dynamic interactions between various lipid components in the membrane. The studies in part A will be experimentally conducted by: a) measuring the zeta potentials of isolated rat myocardial cells and isolated sarcolemmal vesicles from such preparations, as a function of cation concentration for different mono- and divalent cation combinations, and using this information to determine surface charge densities, cation screening vs. adsorption, and the selectivity series of cation binding. b) determining mono- and divalent cation binding constants to isolated sarcolemmal vesicles by analyzing the effects of these cations on vesicle aggregation, using the procedure of Ohki et al. (1982). The studies in part B will be conducted by analyzing the lipid composition of the isolated sarcolemmal vesicles, and then employing this information to examine changes in cation/sarcolemmal interactions using isolated sarcolemmal vesicles having enzymatically modified lipid compositions and reconstituted sarcolemmal vesicles generated from different subgroups of sarcolemmal lipids. By elucidating these mechanisms we hope to provide a firm physical-chemical foundation for understanding how Ca2+-sarcolemmal interactions are involved in regulating E-C coupling.